This invention concerns a continuous process in which hydrogen chloride (HCl) is oxidized to produce a chlorine (Cl2) gas product.
Various processes in the chemical industry consume Cl2 as a feedstock or intermediary and produce HCl as a by-product. Millions of tonnes of HCl are produced annually around the world in this manner. Usually, the chemical producer has no immediate use for the HCl. The required disposal of this waste HCl may be an economic burden. There is a need for a process in which by-product HCl is oxidized to Cl2 for reuse as a chemical feedstock. To be implemented in the chemical industry, such a process will need to be operationally robust, efficient in its overall energy use and economical.
Chien et al. U.S. Pat. No. 3,950,501 discloses a continuous process for oxidizing HCl to Cl2. The process contacts, in a reactor, HCl gas with a mixed acid stream. The term xe2x80x9cmixed acidxe2x80x9d refers to an aqueous mixture of sulfuric and nitric acid. The nitric acid reacts with the HCl according to the reaction:
xe2x80x832HCl+HNO3xe2x86x92Cl2+HNO2+H2O
High purity Cl2 is produced. The sulfuric acid, in the mixed acid, acts as a dehydrating agent and absorbs the water produced by the reaction. The mixed acid is then referred to as spent acid, the connotation being that the absorbed water will need to be removed before the mixed acid can be reused in the process. Nitrous acid produced by the reaction is thought to react with the sulfuric acid to form nitrosyl sulfuric acid: H(NO)SO4.
In the Chien et al. process, the water produced as a result of the HCl oxidation is removed from a minimized split stream of spent acid, typically no more than 10% of the entire spent acid stream. First, this split stream of spent acid is stripped by air or oxygen entered into the process. Nitrosyl sulfuric acid is converted to nitrous acid, which decomposes into NO2 and travels with the stripping gas into a nitric acid regenerator. The stripped sulfuric acid is sent to a concentrator where water is removed to obtain a sulfuric acid strength in the range of 93% to 98%. All concentrations are in wt % unless stated otherwise. The majority of the spent acid is delivered directly to a nitric acid regenerator in which HNO2 is converted to HNO3 through contact with oxygen gas.
There are several inefficiencies inherent in the Chien et al. process. One drawback is that during normal operation, the unit operations of the process may not be stable, resulting in either unoxidized HCl in the product Cl2 or a loss of nitric acid from the process. To be efficient, the Chien process must maintain an exact 1:2 stoichiometric ratio of HNO3 to HCl in the HCl oxidizer. It is impractical to regulate this ratio exactly. When excess HCl is present in the HCl oxidizer, the excess HCl will not be oxidized and will be carried out of the process in the Cl2 product. Conversely, when excess nitric acid is present in the HCl oxidizer, the excess nitric acid travels with the spent acid. A portion of the nitric acid in the spent acid split stream is lost from the process via the acid concentrator.
Also, the Chien et al. process does not use energy efficiently. The significant energy released in the regeneration of the nitric acid cannot be easily or fully employed in reconcentrating the spent sulfuric acid split stream due to the process requirements of reconcentrating 93% to 98% sulfuric acid. Concentrating sulfuric acid to such strengths is energy intensive, requiring a high temperature acid concentrator.
This invention provides a process for converting HCl into Cl2. In preferred embodiments of the invention, it is to be operated as a continuous process. The process exhibits robust and stable operation, reduced nitric acid losses, efficient process energy integration and does not require sulfuric acid to be highly concentrated.
The process involves a sequence of three principal steps or stages.
In the first step, HCl is reacted with an aqueous oxidizing mixture of nitric and sulfuric acids, producing Cl2 gas and water in a stream of spent acids. In addition, this step includes drawing off the Cl2 for further treatment. The spent acid stream consists of nitrous acid (reduced nitric acid), sulfuric acid and water.
The second step regenerates the nitric acid. The nitrous acid, in the stream of spent acids, is oxidized by air or oxygen entering the process.
The third step involves the removal of water, created in the first step of the process, from the spent nitric and sulfuric acid stream. Preferably, the entire stream from the nitric acid regeneration step is used in this re-concentration step. This minimizes the concentration at which the sulfuric acid is re-concentrated. Highly reconcentrated sulfuric acid and the corresponding high temperatures necessary to effect such concentration, are not required.
In step three, the nitric acid is recovered for reuse in the process. This allows excess nitric acid to be used in process step one, achieving robust and stable process operation (complete oxidation of HCl), without the loss of nitric acid from the process.
As the stream of spent acid is reconcentrated after both the HCl oxidation and nitric acid regeneration steps, the heat released from both these reaction steps can be efficiently used in the reconcentrating of the spent mixed acid stream.
Further features and advantages of the invention are described below.